Cationic guar derivatives are known since the early '70s, when their use is cited in the production of waterproof paper (see U.S. Pat. No. 3,589,978).
The first use of cationic guar derivatives in cosmetics goes back to 1977, when a cationic derivative of guar was used in the preparation of a so-called “two in one” shampoo, having hair conditioning characteristics beside the normal detergent power (see U.S. Pat. No. 4,061,602).
The cationic guar derivative used in cosmetic is known with the INCI name of Guar Hydroxypropyltrimonium Chloride and, chemically, is guar 2-hydroxy-3-(trimethylammonium)propyl ether chloride.
Its synthesis, described as an example in the cited
U.S. Pat. No. 3,589,978, requires the reaction of 2,3-epoxypropyl trimethylammonium chloride or (3-chloro-2-hydroxypropyl) trimethylammonium chloride on the hydroxyl groups of guar, in the presence of basic catalysts (such as sodium hydroxide).
The reagent 2,3-epoxypropyl trimethyl ammonium chloride is classified as a carcinogenic substance CMR 2; the reagent 3-chloro-2-hydroxypropyl trimethylammonium chloride, even if less toxic than the corresponding epoxide, is classified as CMR 3 and in alkaline medium is converted into 2,3-epoxypropyl trimethyl ammonium chloride.
According to what is described in U.S. Pat. No. 3,589,978, the reaction can be carried out in a solvent such as isopropanol, methanol, ethanol and tert-butanol, at temperatures between 30 and 60° C.; in the Example A of the same patent, at the end of the reaction and after neutralisation of the alkali excess, the product is dried, milled and washed with methanol, in the attempt to eliminate the un-reacted quaternary reagent.
Nothing is reported in U.S. Pat. No. 3,589,978 on the residual quantities of the cationic reagent (2,3-epoxypropyl trimethyl ammonium chloride) in the final product; but we can observe that methanol is itself a toxic product.
In U.S. Pat. No. 4,031,307 the preparation of cationic derivatives of guar in a biphasic system is described, by reacting solid guar with a cationising reagent in a mixture of water and water soluble solvent that contains the basic catalyst; after the reaction, the obtained product is separated by centrifugation or filtration and preferably purified by means of a first washing with the water-solvent mixture used in synthesis, and with a second washing with a more anhydrous form of the same solvent.
In US 2001051143, the preparation of guar cationic derivatives comprising at the end of the reaction a first washing with 85% by weight aqueous isopropanol and a second washing with pure isopropanol is described; in US 2001051140 the preparation of guar cationic derivative comprising, at the end of the reaction, two washings with 85% aqueous isopropanol is described.
In all these publications, neither the type nor the amount of impurities present in the obtained cationic guar derivative is mentioned, impurities that will become part of the final cosmetic product in case the cationic guar derivative is used as a cosmetic raw material.
The greatest part of cationic guar derivatives currently on the market, in order to obviate to the difficulty and to the burden connected with the purification by washings with water and solvents, are purified by washing the product, previously cross-linked with borates, with water only, as described for example in CA 2,023,324, where in the reaction phase borax is added; the cationic guar derivatives so obtained contain small amounts of boron (borated guars).
The aim of the cross-linking with boric acid is to form bonds, through the borate anion, between the polysaccharides chains, that render the product insoluble in water.
These bonds are stable at basic pH, and therefore in such conditions the product can be washed from by-products.
In acidic conditions the bonds with the borate are removed, the product is soluble and can perform its thickening and conditioning properties.
The reaction with borates is reversible with pH changes; therefore, even if the product is pre-solubilised at acidic pH, bringing it back to alkaline conditions may cause a change in the viscosity of the formulate, varying as a consequence the quality of the product.
The limit of borated cationic guar derivatives is therefore their exclusive applicability to products to be used at acidic or slightly acidic pH, because products treated with borates are not soluble at pH greater than 7.
Furthermore, it must be observed that boric acid derivatives are classified as substances toxic for reproduction of category CMR 2.
Hair dyes represent one of the sector of the home & personal care in the greatest expansion; they are generally formulated at pH higher than 8 and therefore it is not possible to formulate them with cationic guar derivatives purified by cross-linking with boric acid.
The preparation of solid soap bars, generally having pH higher than 7.5 in aqueous solution and normally containing softening and conditioning agents for the skin, represent another field of great interest for the use of non borated cationic guar derivatives.
The preparation of depilatory creams and the production of fabric powder detergents, having generally basic pH and possibly advantageously containing a co-formulating agent having skin protective function, are further formulations in which non-borated cationic guar derivatives can be used.
The Applicant has now found a procedure for the preparation of purified cationic guar, soluble at all pHs, free from boron and with a reduced content of other impurities that can compromise the stability or the toxicological characteristics of the cosmetic formulations in which it is used as an ingredient.